The 3rd Generation Partnership Project (3GPP) Long Term Evolution, usually referred to as LTE, is a standard for wireless communication of high-speed data for mobile phones and data terminals, which is based on the GSM/EDGE and UMTS/HSPA network technologies. It is designed to better support mobile broadband Internet access by increasing the capacity and speed using new modulation techniques. Although commonly referred to as a type of 4G wireless service, LTE release 8, which is currently in use does not satisfy the requirements set forth by the ITU-R organization. Future releases of LTE (referred to as LTE Advanced) are expected to satisfy the requirements to be considered 4G.
Sounding techniques are commonly used in the uplink of LTE or LTE-A systems by the radio base stations, usually known as eNB or eNodeB, (where “e” stands for evolved) to estimate the uplink channel state at different frequencies. The channel-state estimates can then, for example, be used by the network scheduler to assign resource blocks of instantaneously good quality for uplink physical uplink shared channel (PUSCH) transmission, as well as to select different transmission parameters such as the instantaneous data rate and different parameters related to uplink multi-antenna transmission. Uplink sounding can also be used by the network to control the uplink transmit timing by means of the uplink-timing-alignment procedure.
In the current LTE or LTE-A systems, there has been designed reference signals that are used for sounding the uplink channel, referred to as Sounding Reference Signal (SRS). SRS is typically wideband in nature, since it is often used to probe a wide range of bandwidth. Base stations can then perform sounding by transmitting SRS to estimate the uplink channel state at different frequencies. Generally, SRS transmission could occur at regular time intervals or in an aperiodic manner.
However, with the development of LTE technology, the uplink sounding solutions with SRS turn out to be not adequate. For example, in LTE-advanced, as specified in 3GPP release 10, SRS resources are insufficient to support timely sounding since uplink channel states related to maximum 4 antennas need to be monitored. In the context of CoMP (coordinated multi-point transmission) in release 11, the problem becomes even worse, considering the SRS resources might be shared by all UEs.
Sounding is an important aspect in the uplink performance enhancement of a wireless communication system, such as the LTE system. To seek an efficient and cost-effective way of sounding, various substitutes for SRS techniques are being considered. However, there is essentially no viable scheme that can well fit in the existing architecture.